1. Field of the Invention
This invention relates to a dry etching method used in the preparation of semiconductor devices. More particularly, it relates to raising the wafer cooling temperature in so-called low temperature etching and to improving selectivity with respect to the underlying layer and to the mask.
2. Description of the Related Art
With the tendency in recent years towards a higher integration degree and higher performance of semiconductor devices, such as VLSIs or ULSIs, a strong demand has been raised for a technique in the field of dry etching whereby requirements for high anisotropy, high etchrate and high selectivity may be achieved simultaneously.
A so-called low-temperature etching, in which etching is performed while the wafer temperature is controlled during etching so as to be not higher than 0.degree. C., is one of the techniques which has attracted attention under such a background. The low-temperature etching is a technique in which a radical reaction on the pattern sidewall is "frozen" or suppressed to prevent profile defects such as undercuts, while the etchrate along the depth is maintained by the ion assist effects by maintaining the wafer at lower temperatures. For example, in a lecture paper number 28a-G-2, page 495, Lecture Papers, in Extended Abstracts of the 35th Spring Meeting of the Japan Society of Applied Physics and related Societies, a report has been made on silicon trench etching and etching of a n.sup.+ type polysilicon layer, in which etching was performed using an SF.sub.6 gas while the wafer was cooled to -130.degree. C.
However, a number of problems remain to be solved in the low-temperature etching if it is to be applied in practice. One of these problems is the wafer cooling temperature. If high anisotropy is to be achieved by "freezing" (termination) or suppression of the radical reaction, a low temperature is required which may be achieved by liquid nitrogen. Thus, cooling to -120.degree. to -130.degree. C. is used for single crystal silicon or polysilicon, while cooling to -100.degree. C. and cooling to about -60.degree. C. are used for a resist material and tungsten silicide, respectively. The result is that a peripheral cooling equipment of a considerable cooling capacity is required and this equipment increases the size of the apparatus and raises production costs. Although it is known to use reaction products for sidewall protection in the mid to low temperature, there is a risk that, due to the low vapor pressure of reaction products, these tend to be deposited on the pattern sidewall to increase the pattern width. In addition, the cooling time or the time necessary for post-heating for preventing dewing after etching tends to be prolonged to detract from economic profitability or throughput. For these reasons, it has been desired to raise the wafer cooling temperature closer to room temperature.
Another problem is the setting of the wafer temperature for etching a layered structure composed of layers of different materials. Since the reaction between etchant(s) and the material to be etched is partially suppressed in the low-temperature etching, the temperature at which high anisotropy is achieved is delicately changed depending on different combinations of the etching gas and the etched material. Thus, although a single temperature setting suffices for a material composed of a single layer, satisfactory anisotropic processing is difficult to achieve by a single temperature setting in the case of, for example, a polycide layer composed of superimposed tungsten silicide and doped polysilicon layers, because the optimum temperatures for low-temperature etching of these two layers differ by as much as about 60.degree. C. Although a multi-chamber etching equipment would be required for stably setting plural temperatures, this is not desirable in view of the space occupied by the equipment and increased running costs and costs for maintaining a clean zone in a clean room.